1. Field of the Invention
The present invention relates to an opposed wiping type wiper unit for wiping a window of an automobile or the like.
2. Description of Related Art
An opposed wiping type wiper unit generally comprises a crank arm that rotates when driven by a motor, an intermediate link pivotally supported by an intermediate link fulcrum, and a connecting rod interposed between the crank arm and the intermediate link so as to convert the rotation of the crank arm into reciprocating oscillation of the intermediate link. In addition, the upper and lower ends of the intermediate link are respectively connected to driving levers of right-hand and left-hand wiper shafts through driving rods so as to cause opposed wiping operations of right-hand and left-hand wiper arms. Further, the angular velocity changes of the right-hand and left-hand wiper arms are set to be asymmetrical so that the wiper blades of the right-hand and left-hand wiper arms are superposed at a lower reversal position. Namely, in the vicinity of the lower reversal position, the angular velocity of the upper wiper arm (on the driver side) is set to be higher, and the angular velocity of the lower wiper arm (on the passenger side) is set to be lower so that the right-hand and left-hand wiper blades are superposed without interference therebetween.
Previously, when the angular velocity changes of the right-hand and left-hand wiper arms were set to be asymmetrical, however, all factors of angular velocity changes, such as the length of the connecting rod, the connecting position of the connecting rod to the intermediate link, the length of the driving rods, the connecting positions of the driving rods to the intermediate link, etc. were variably set. For example, all of these dimensions had to be changed when the vehicle has the driver position on the left-hand side instead of on the right-hand side. This not only complicates setting but also causes the need to design wiper units according to different setting specifications for different vehicle types and driver positions (i.e., right-hand and left-hand specifications). As a result, the parts to be used are changed in accordance with the specifications, and the number of part types is significantly increased, thereby increasing the cost.
As shown in FIG. 11, when a crank arm 3 and a connecting rod 5 are on a line at both the lower and upper reversal positions, and when both reversal positions A and B are set (uniform distribution setting) to be on this line, the intermediate position C of the crank arm 3 in the forward wiping motion of the wiper and the intermediate position C' in the backward wiping motion are not at symmetrical opposite positions at 180", but are slightly shifted from the symmetrical positions (at a calculated angle of 167.29') from the geometrical viewpoint of conversion from rotation into oscillation as shown in FIG. 11. For the same reason, the oscillation angle of the driving lever 7 at a position before the reversal (upper reversal) position where the crank arm 3 rotates for 200 in the forward motion is slightly different (a calculated value of 0.91.degree.) from the oscillation angle in the backward motion.
Actual changes in the angular acceleration are described below with reference to FIG. 9 (an embodiment of the present invention employing uniform distribution setting for comparison). The changes in the operating angle show a sine wave with a maximum at the upper reversal position, and the changes in the angular velocity also show a sine wave form that is point-symmetrical about the upper reversal position. On the other hand, the changes in the angular acceleration in the forward and backward wiping motions of the wipers are not symmetrical for the above reason and due to the stress of the pushing and drawing motion of the connecting rod on the driving lever. The angular acceleration becomes maximum immediately before the upper reversal position and lower reversal position, and the absolute values of both maximum angular accelerations are different.
The velocities of the forward and backward wiping movements of the wipers are thus different from each other. Particularly, in an opposed wiping type wiper unit in which right and left wiper arms are superposed at the center, setting is complicated for the above reasons, and it is difficult to calculate an ideal wiping pattern and produce e wiper unit.
In summary, the setting of a conventional opposed wiping type wiper unit must be strictly balanced in order for the right and left side wiper arms to move without interfering each other when they reach the housing position. In the uniform distribution setting shown in FIG. 9, the maximum angular acceleration changes are properly set to be 0.6 rad/s.sup.2 slightly before the housing position, i.e., 360.degree.. If the same motor is used, i.e., a rotating direction is in the same direction, for both a right and left hand specification car, a 180.degree. phase difference must be implemented because the moving direction of the wiper arms for the right hand specification is opposite to that of the left hand specification. Namely if a wiper unit is used for the reverse hand specification, the housing position of the wiper arms corresponds to 180.degree. of FIG. 9 of which minimum angular acceleration changes are -0.9 rad/s.sup.2. An absolute value difference of 0.3 rad/s.sup.2 is created, which causes an imbalance such that the right and left hand wiper blades interfere each other.
Thus, theoretically, with the uniform distribution setting, the same wiper motor cannot be used commonly for both the right hand specification and the left hand specification because all components, including the wiper motor, must be produced reflecting the symmetrical relationship based upon 0.degree. in FIG. 9 in accordance with the difference between right and left hand specifications. Therefore, components for the right hand specification are symmetrically different from components for the left hand specification, however, the same dimensional setting is applied to both specifications. On the other hand, if a motor with the same rotating direction is used for both specifications, e.g., as is common in actual production, the size of the components of the motor is set to be different for the right hand specification as opposed to the left hand specification.